Gas discharge ultraviolet lasers used as a light source for integrated circuit lithography typically are line narrowed. A preferred line narrowing prior art technique is to use a grating based line narrowing unit along with an output coupler to form the laser resonance cavity. The gain medium within this cavity is produced by electrical discharges into a circulating laser gas such as krypton, fluorine and neon (for a KrF laser); argon, fluorine and neon (for an ArF laser); or fluorine and helium and/or neon (for an F2 laser).
A sketch of such a prior art system is shown in FIG. 1 which is extracted from Japan Patent No. 2,696,285. The system shown includes output coupler (or front mirror) 4, laser chamber 3, chamber windows 11, and a grating based line narrowing unit 7. The line narrowing unit 7 is typically provided on a lithography laser system as an easily replaceable unit and is sometimes called a xe2x80x9cline narrowing packagexe2x80x9d or xe2x80x9cLNPxe2x80x9d for short. This unit includes two beam expanding prisms 27 and 29 and a grating 16 disposed in a Litrow configuration. Gratings used in these systems are extremely sensitive optical devices and deteriorate rapidly under ultraviolet radiation in the presence of oxygen in standard air. For this reason, the optical components of line narrowing units for lithography lasers are typically purged continuously during operation with nitrogen.
For many years, designers for line narrowed lasers have believed that distortions of the laser beam could be caused by gas flow near the face of the grating. Therefore, laser designers in the past have made special efforts to keep the purge nitrogen from flowing directly on the face of the grating. Several examples of these efforts are described in the Japan Patent 2,696,285 referred to above. In the example shown in extracted FIG. 1, the purge flow is directed from N2 gas bottle 44 toward the back side of grating 16 through port 46.
Another prior art excimer laser system utilizing a diffraction grating for spectrum line selection is shown in FIG. 2. The cavity of the laser is created by an output coupler 4 and a grating 16, which works as a reflector and a spectral selective element. Output coupler 4 reflects a portion of the light back to the laser and transmits the other portion 6 which is the output of the laser. Prisms 8, 10 and 12 form a beam expander, which expands the beam before it heats the grating. A mirror 14 is used to steer the beam as it propagates towards the grating, thus controlling the angle of incidence. The laser central wavelength is normally changed by tuning that mirror 14. A gain generation is created in chamber 3.
Diffraction grating provides the wavelength selection by reflecting light with different wavelengths at different angles. Because of that only those light rays, which are reflected back to the laser, will be amplified by the laser gain media, while all other light with different wavelengths will be lost.
The diffraction grating in this prior art laser works in a so-called Littrow configuration, when it reflects light exactly back. For this configuration, the incident (diffracted) angle and the wavelength are related through the formula:
2d n sin xcex1=mxcexxe2x80x83xe2x80x83(1)
where xcex1 is the incidence (diffracted) angle on the grating, m is the diffraction order, n is refractive index of gas, and d is the period of the grating.
Because the microlithography exposure lens is very sensitive to chromatic abberations of the light source, it is required that the laser produce light with very narrow spectrum line width. For example, state of the art excimer lasers are now produce spectral linewidth on the order of 0.5 pm as measured at Full Width at half maximum values and with 95% of the light energy concentrated in the range of about 1.5 pm. New generations of microlithography exposure tools will require even tighter spectral requirements. In addition, it is very important that the laser central wavelength be maintained to very high accuracy as well. In practice, it is required that the central wavelength is maintained to better than 0.05-0.1 pm stability. The state of the art microlithography excimer laser does have an onboard spectrometer, which can control the laser wavelength to, the required accuracy. The problem is, however, that in order for that spectrometer to work, the laser must be firing pulses. Therefore, when the laser is continuously exposing the wafers, its spectrometer can control the wavelength to the required accuracy. The problem arises, when the exposure process is stopped, such as for wafers replacement. The wafer replacement may take a minute or two, and during that time the laser is not allowed to fire pulses. When the laser is firing, it produces a lot of heat. When the laser is not firing, it cools down. This cooling down can change the laser wavelength due to thermal drifts. One of the possible causes for the drift is change in the refractive index n of the gas with temperature, according to the above equation. This change in n will cause change in Littrow wavelength of the grating, and therefore, change the laser operating central wavelength. Therefore, the first several pulses after the laser resumes firing will often be at a different wavelength than required. If these pulses are used to expose wafers, the chromatic aberration will cause the quality of the image to degrade. That in turn may cause severe yield issues. One solution to the problem is not to use these first few pulses for wafer exposure. This can be done by closing the mechanical shutter of the laser during the first pulses. Unfortunately, because closing and opening of the mechanical shutter takes time, it will cause the throughput reduction. The lithography laser works in tandem with a number of very expensive tools in a semiconductor fab. Therefore, even a 1% reduction in the throughput of the laser will bear a substantial price tag.
Line narrowed ultraviolet laser light sources currently in use in the integrated circuit industry typically produce about 10 mJ per pulse at repetition rates of about 2000 Hz and duty factors of about 20 percent. Increased integrated circuit production can be achieved at higher repetition rates and greater duty cycles. Applicants and their fellow workers have designed and tested a 4000 Hz gas discharge lithography laser. Applicants are now experimenting with even higher repetition rates and are attempting to minimize laser center wavelength drifts. Applicants have experienced difficulties maintaining consistent narrow bandwidths at these higher repetition rates and duty cycles.
A need exists for reliable line narrowing devices and techniques for high repetition rate, high duty cycle gas discharge lasers.
The present invention provides helium purge for a grating based line narrowing device for minimizing thermal distortions in line narrowed lasers producing high energy laser beams at high repetition rates. Applicants have shown substantial improvement in performance with the use of helium purge as compared to prior art nitrogen purges.
In preferred embodiments a stream of helium gas is directed across the face of the grating. In other embodiments the purge gas pressure is reduced to reduce the optical effects of the hot gas layer.